Image suppressor



Oct. 7, 1941. Hfc. FORBES 2,258,137

IMAGE SUPPRES S OR Filed April 15, 1940 INVENTOR Have) 6: P024955 ATTORNEY lator.

.the standpoint .of material and Patented Oct. 7, 1941 IMAGE sUPPREssoR" Henry C. Forbe half to 0010 N. Y., and one-half to Th pany, Springfield, Mass.

nia'l Radio Corporation,

s, Buffalo, N.-*Y., assign'or-of one- Buffalo, eLF'. W. 'Sicklcs .Co'm- Application April 1-3, 1940,'ISerial N0. '3 29, 461

11'Claims. (Cl.'25020) This invention relates to radio receiving apparatus, and more particularly to signal selecting circuits for superheterodyne receivers.

In such receivers, as is Well known, the intermediate frequency amplifying stage or stages are tuned to a frequency above audibility and usually below the lowest frequency of the broadcast band. Incoming radio frequency signals are heterodyned or beat with an oscillation produced'by the local oscillator, and this gives rise to oscillations of a frequency which may be either the sum of or the difference between the incoming frequency and that of the local oscil- Only those oscillations thereby produced which have a fre through the intermediate fr are then amplified.

For example, if the intermediate frequency amplifier is tuned to 400 kilocycles and a 550 kilocycle signal is being received, and the local oscillator has, when this particular signal is being received, a frequency of 950 kilocycles, then there will be produced a beat frequency of 400 kilocycles. However, if there is also an incoming signal of 1350 kilocycles at the same time, a new oscillation will be produced by the heterodyne action between the local oscillation and the 1350 kilocycle signal, and this new oscillation will likewise have a frequency of 400 kilocycles and will accordingly pass through the intermediate frequency amplifier, resulting in interference and cross-talk with the desired signal. The frequency of theundesired signal is termed the image frequency, and it is equal to the frequency of the desired si intermediate frequency. I

It is usual to provide a signal selecting .circuit in advanceof thefirst detector, and this circuit is ordinarily arranged to be tuned to the frequency of the desired signal. However, even when such a circuit is used, image interference may still be encountered and it has vbeen proposed in the past to elminate or reduce this image-interferenceby providing preselecting circuits and the like, andalso by providing one or equency amplifier more wave traps associated with the antenna and variably tuned.

The employment of wave" traps for image suppression increases the cost of the receiver, since it is necessary to provide. an additional coil and an additional condenser, one or the other 'of which is usually variable. Theprovision of such parts increases the cost of the receiver, from parts employed and also the labor of assembly.

quency enabling them to passgnal plus twice the Ihave discovered that it is possible toprovide asimple tuning circuit which acts simultaneous- 'ly as a-selector of desired frequencies and as an i age suppressor; that is to say, the variation of' the variable :react'ances :causes the circuit to select the desired signal, at the same time to present an extremely high impedance to the image signal, and I have found that circuits "according to my invention produce signal to image ratios as high as, and in many cases considerably higher .than, the best signal. to image ratios produced by conventional wave trap circuits. As anexample, employing the circuit of 'my invention, r-have been able to obtain signal to image ratios as high as 70,000.

"Among the objects-of my invention are:

To provide an image suppression circuit which does not require separate wave trap or preselector circuits; I

To provide a circuit in which the image suppression is automatically performed by the main tuning circuit.

Still another object- 'of-my invention is to provide a simple signal selectingcircuit which will deliver signal to image ratios as high orhigher than those obtained from conventional wave trap or preselector image suppressors.

Still otherobjects of my invention will be apparent from the specification.

In this application I have particularly pointed out and distinctly claimed the part, improvement :or combination which I claim as my invention *or discovery and I have explained theprinciples thereof and the best mode in which I have contemplated'applying those-principles so as to'distinguish my invention from other inventions.

In the drawing- The .figureis a circuit 'diagramof one form of circuit in accordance with my invention.

In the circuit diagram, I represents an an- .tenna of :any suitable type which may be connected to ground through coupling'condenser 2. Currents produced'in the antenna I by incoming signals'williiow to some extent through inductance3, shunted by condenser -5, and the'voltage thereby produced may be supplied to the grid of the first tube 7, which-may be connected to ground through condenser 6, and the input circuitmaybe tuned as a whole to select desired "signals by 'means of tuning condenser 10 in parallel with trimmer condenser 6 connected from the grid'of tube 1 to ground. A1The antenna side of coil 3 may be connected through resistance H, having a high resistance to ground or to the automatic volume control voltage source, if such is present, thereby providing a direct current grid return path. The cathode of tube 1 may be grounded through resistance l2, sh unt ed by condenser I3, or directly grounded if desired.

Since my invention is not concerned with the construction of the superheterodyne receiver beyond this point, and the same is well-known in the art, only the circuits ahead of the first tube are shown and described.

If now condenser 5 be connected across coil 3 and chosen of the proper value, a very greatimprovement in signal to image ratio is noticed. In choosing the value of condenser 5, it is picked of such magnitude as to cause the inductance 3 to resonate with condenser 5 at the image frequency (which, as stated, is equal to the desired signal frequency, plus twice the intermediate frequency) when the tuning circuit is tuned to a desired signal frequency.

The connection of this condenser 5, which will ordinarily be quite small, for instance of the order of two to twelve m. m. f., may slightly disturb the tracking of the main tuning circuit with the oscillator tuner, in which case it may be necessary to retrim the main tuning circuit by variation of the trimmer condenser 5, but it will be found that when the operation is completed the entire input circuit,'as a whole, is resonant to the desired signal frequency; whereas, inductance 3 and condenser 5, forming a closed loop circuit, are resonant to the image-producing signal and cause extreme attenuation thereof while not interfering with the selection by resonance of the desired signal. I have found that movement of the tuning condenser l varies the tuning of the entire circuit and also the tuning of the image suppressor 3-5 in a satisfacteory approximation to the desired manner. g j,

In order for the signal or main tuning circuit to vary from 500 to 1500 kilocycles, i. e., 3 to 1 ratio, the capacity of that circuit must vary in a 9 to 1 ratio. It will be noted, howeven that while the resonant circuit varies from 500 to 1500 kilocycles, the image frequency varies from 1460 to 2410 kilocycles when the intermediate frequency is 455 kilocycles. In this case the ratio of frequency change is 1.72 to 1 and the change of capacity required to make the image circuit track exactly should be in the square of this ratio, or 2.96 to 1. If condenser is left set at the optimum adjustment, made as already described, the resonant frequency of the closed loop circuit consisting of inductance 3 and condenser 5 will remain fixed as the tuning condenser 10 tunes the input circuit as a whole to higher frequencies, while the image frequency rises.

This departure from the theoretical image value is, in most cases, not a serious matter, because, ordinarily, the most harmful image interference is obtained when the main tuning circuit is tuned to the low frequency end of the broadcast band. However, in case this departure from the theoretical value of the image trap is desired to be reduced or eliminated, it may be done as described hereinafter.

In the case where varied while the receiver is in use by the purchaser, and which can maintain the image trap circuit in resonance with the image frequency for substantially only a small part of the broadcast band, the main tuning circuit would be tuned to thepoint at which the worst image interference isexperienced, and the image trap 3-5 tuned to the image frequency at this point by variation of condenser 5 is not to be condenser 5, which would then be left set at its best value.

The image trap may be made to track closely with the image frequency by ganging condenser 5 with condenser l0 and the oscillator tuning condenser, the values of condenser 5 required being calculated, and the number and shape of the plates being so chosen as to produce the required capacity at a number of points distributed over the tuning range. Following the example previously given, if the intermediate frequency is 455 kilocycles, condenser I0 would give a change of capacity of 9 to 1, whereas, condenser 5 would give a ratio of 2.96 to 1, or 3 to 1 approximately, and variation of condenser 5 would not substantially interfere with the tuning of the main tuning circuit, because its maximum value is still too low to be an effective shunt around inductance 3.

It may be noted that under certain conditions the value of condenser 2 may be zero; that is, that physical condenser may be omitted. One instance of this is in automobile radio installations where the running-boards are used as an aerial. In this instance, the antenna capacity, which is in parallel with condenser 2 is so large, comparatively, that condenser 2 may be omitted.

Condenser 2 may be omitted with an aerial of small capacity also, such as a fishpole. In this instance condenser 6 may be adjusted to compensate for the variation in antenna capacity.

While I have shown and described certain preferred embodiments of my invention, it will be apparent that modifications and changes may be made without departing from the spirit and scope of my invention, as will be clear to those skilled in the art.

I claim:

1. In a superheterodyne receiver, in combination, an inductance element and a plurality of capacity elements including a tuning capacity element for selecting desired signals, connected to gether to form a resonant signal selector circuit for connection to one point to an antenna and at another point to the control electrode of a thermionic tube, said inductance having one of said capacity elements connected in shunt therewith and forming with said inductance a resonant circuit parallel tuned to the image of a selectable signal frequency, said parallel tuned circuit being interposed in series between the points of connection to said antenna and to said grid.

2. In a superheterodyne receiver, in combination, a combined signal selector and image rejector circuit, comprising an inductance, a tuning capacity connected therewith for selecting desired signals, and a second capacity connected in shunt with at least a portion of said inductance, said second capacity and said inductance forming a circuit parallel tuned to the image of a selectable signal frequency, the impedance of said second capacity being so great, that said capacity does not form an effective shunt around said iriductance at selectable signal frequencies, said circuit having one point thereon to be connected to an antenna, and another point to be connected to the control electrode of a thermionic tube, and said parallel tuned circuit being interposed in series between said points.

3. In a'superheterodyne receiver, an input circuit comprising a pair of capacity elements, a connection between said capacity elements, said connection including in series a closed tuned circuit, the inductance and capacity of said closed tuned circuit being so chosen that said closed tuned circuit is resonant to the image of a selectable frequency and one of the reactance elements of said input circuit external to said closed tuned circuit being variable over a range such as to tune the input tuning circuit as a whole to desired signals, said input tuning circuit having a pair of points, one to be connected to an antenna, and the other to the control grid of a thermionic tube, and said closed tuned circuit being interposed in series between said points.

4. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, in-

c'luding a first resonant circuit a portion of which is connected between said control electrode and ground, and a parallel resonant circuit in series between said antenna and said control electrode, said resonant circuits being tuned by capacity variation respectively to a signal frequency and to the corresponding image frequency.

5. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connected between said control electrode and ground, and a parallel resonant circuit in series between said antenna and said control electrode, said resonant circuits including in common an inductance and means for tuning said first resonant circuit to desired signal frequencies and tuning said second resonant circuit to the image of a desired signal frequency, said means including a first capacity in shunt with said inductance, and a second capacity in said first circuit, said latter capacity being variable to tune said first circuit over the desired frequency range.

6. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connected between said control electrode and ground, and a parallel resonant circuit in series between said antenna and said control electrode, said resonant circuits including in common an inductance, a first capacity forming with said inductance a closed loop circuit, and a second capacity in said first circuit, said latter capacity being variable to tune said first circuit over the desired frequency range, and the value of said first capacity being such as to tune said closed loop circuit to the image of a frequency within the desired frequency range.

7. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connected between said control electrode and ground, and a parallel resonant circuit in series between said antenna and said control electrode, said resonant circuits including a common inductance, a capacity shunting said inductance and forming therewith said second resonant circuit and a second capacity in said first circuit, both said capacities being variable, and the maximum value of said first capacity being relatively small as compared with the maximum value of said second capacity.

8. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connected between said control electrode and ground, and a parallel resonant circuit in series between said antenna and said control electrode, said resonant circuits including a common inductance, a capacity shunting said inductance and forming therewith said second resonant circuit and a second capacity in said first circuit, both said capacities being variable, and the maximum value of said first capacity being relatively small as compared with the maximum value of said second capacity, and means for varying both said capacities simultaneously.

9. An image suppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connected between said control electrode and ground, and a parallel resonant circuit in series between said antenna and said control electrode, said resonant circuit including a common inductance, a capacity shunting said inductance and forming therewith said second resonant circuit and a second capacity in said first circuit, both said capacities being variable, and the maximum value of said first capacity being relatively small as compared with the maximum value of said second capacity, and a single means for simultaneously varying both said capacities in the same sense, but at different rates.

10. An image suppression system for a superheterodyne radio receiver having an antennaand a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connected between said control electrode and ground, and a parallel resonant circuit in series between said antenna and said control electrode, said resonant circuits including a common inductance, a capacity shunting said inductance and forming therewith said second resonant circuit and a second capacity in said first circuit, both said capacities being variable, and the maximum value of said first capacity being relatively small as compared with the maximum value of said second capacity, and a single means for simultaneously varying both said condensers in the same sense, said condensers having their capacity values for different settings so chosen that when said first resonant circuit is tuned to select a desired signal frequency said second circuit is tuned to the image thereof.

11. An imagesuppression system for a superheterodyne radio receiver having an antenna and a vacuum tube with a control electrode, including a first resonant circuit a portion of which is connected between said control electrode and ground, and a parallel resonant circuit in series between said antenna and said control electrode, said resonant circuits including a common inductance, a capacity shunting said inductance and forming therewith said second resonant circuit and a second capacity in said first circuit, both said capacities being variable, and the maximum value of said first capacity being relatively small as compared with the maximum value ofsaid second capacity, and a single means for simultaneously varying both said condensers in the same sense, the second capacity varying according to where in and ii. are respectively the highest and lowest frequencies to be selected by said second capacity, and the first capacity varying according to H+2f1 L+ f1 where f: is the intermediate frequency.

HENRY C. FORBES.

CERTIFICATE OF CORBECTION Patent No. 2,2 ,157. October 7, 19in.

HENRY C. FORBES.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, first column, line 58, for sati sfacteory read -sati sfactory-; and second column, line 15, claim 1, for the words to one" read -at one; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Officen Signed and sealed this 11th day of November, A. D. l9lpl.

Henry Van Arsdale,

(Seal) Acting; Commissioner of Patents 

